Embodiments according to the invention are related to an apparatus for generating a representation of a bandwidth-extended signal on the basis of an input signal representation. Other embodiments according to the invention are related to a method for generating a representation of a bandwidth-extended signal on the basis of an input signal representation. Further embodiments according to the invention are related to a computer program for performing such method.
Some embodiments according to the invention are related to novel patching methods inside spectral band replication.
Storage or transmission of audio signals is often subject to strict bitrate constraints. These constraints are usually overcome by a coding of the signal. In the past, coders were forced to drastically reduce the transmitted audio bandwidth when only a very low bitrate was available. Modern audio codecs are nowadays able to preserve the audible bandwidth by using bandwidth extension (BWE) methods. Such methods are described, for example, in references [1] to [12]. These algorithms rely on a parametric representation of the high-frequency content (HF), which is generated from the waveform-coded low-frequency part (LF) of the decoded signal by means of transposition into the HF spectral region (“patching”) and the application of a parameter driven post processing.
In the art, methods of bandwidth extension, such as spectral band replication (SBR) are used as an efficient method to generate high-frequency signals in HFR (high-frequency reconstruction) based codecs.
The spectral band replication described in reference [1], which is also briefly designated as “SBR”, uses a quadrature mirror filterbank (QMF) for generating the HF information. With the help of the so-called “patching” process, lower QMF-bands are copied to higher (frequency) position yielding in a replication of the information of the LF part in the HF part. The generated HF is afterwards adapted to the original HF part with the help of parameters that adopt (or adjust) the spectral envelope and the tonality (for example using an envelope formatting).
In standard SBR, patching is carried out by a copy operation inside the QMF-domain. It has been found that this can sometimes lead to auditory artifacts, particularly if sinusoids are copied into the vicinity of each other at the border of LF and the generated HF part. Thus, it can be stated that the standard SBR has the problem of auditory artifacts. Also, some conventional implementations of bandwidth extension concept bring along a comparatively high complexity. Additionally, in some invention implementations of bandwidth extension concepts, the spectrum becomes very sparse for high patches (high stretching factors), which may result in undesired (audible) audio artifacts.
In view of the above discussion, it is an objective of the present invention to create a concept for generating a representation of a bandwidth-extended signal on the basis of an input signal representation, which brings along an improved tradeoff between complexity and audio quality.